Group III-V Ferromagnetic/Non-magnetic Semiconductor Heterojunctions and Magnetodiodes

ABSTRACT

Ferromagnetic Group III-V semiconductor/non-magnetic Group III-V semiconductor heterojunctions, with a magnetodiode device, to detect heterojunction magnetoresistance responsive to an applied magnetic field.

This application is a divisional of and claims priority to and thebenefit of application Ser. No. 13/097,688 filed Apr. 29, 2011 andissued as U.S. Pat. No. 9,024,370 on May 5, 2015, which was a divisionalof application Ser. No. 11/476,253 filed Jun. 27, 2006 and issued asU.S. Pat. No. 7,956,608 on Jun. 7, 2011, which claimed priority benefitfrom application Ser. No. 60/694,420, filed Jun. 27, 2005, each of whichis incorporated herein by reference in its entirety.

This invention was made with government support under grant numberDMR-0076097 awarded by the National Science Foundation and grant numberECS-0224210 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Magnetic sensors and magnetoresistive devices have been utilized in awide variety of applications. However, such devices have, for the mostpart, utilized conventional semiconductors and are restricted in termsof potential use and development due to inherent limitation of suchmaterials. For instance, non-linear response to an applied magneticfield limits the scalability of such devices and places practicallimitations on size reduction. Likewise, magnetoresistance inconventional magnetodiodes is dependent upon transverse magnetic fields.Such a limitation has typically required use of at least two suchdevices for multi-directional sensing.

As a result, the search for an alternate approach has been an on-goingconcern in the art. Accordingly, the development of ferromagnetic(III-V) semiconductors has generated much interest in the possibility ofnew magnetoelectronic devices and all-semiconductor spintronic logicdevices. Despite the technological potential of such heterojunctions,little is known about their junction current-voltage characteristics.Even less is known about the magnetoresistance of these heterojunctions.Models have been developed for ideal junction characteristics in amagnetic field, but are based on diffusion of carriers over a potentialbarrier. Such considerations are, at least in part, due to conventionalviews of such device structures and current III-V semiconductors; thatis, tunneling processes are responsible for forward currentcharacteristics, especially at low temperatures and in heavily dopedjunctions.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide Group III-V ferromagnetic/non-magnetic semiconductorheterojunctions, composites, articles and/or device structures, togetherwith methods for their use, thereby overcoming various deficiencies andshortcomings of the prior art, including those outlined above. It willbe understood by those skilled in the art that one or more aspects ofthis invention can meet certain objectives, while one or more otheraspects can meet certain other objectives. Each objective may not applyequally, in all its respects, to every aspect of this invention. Assuch, the following objects can be viewed in the alternative withrespect to any one aspect of this invention.

It can be an object of the present invention to provide amagnetoresistive device providing positive magnetoresistance,notwithstanding strength or direction of applied magnetic field, usingp-type and n-type materials of the sort described herein.

It can be another object of the present invention to provide, inconjunction with the preceding objective, a range offerromagnetic/non-magnetic semiconductor heterojunctions exhibitinglinear magnetic response.

It can be another object of the present invention to provide, inconjunction with one or more of the preceding objectives, devicestructures and/or method(s) for their use, without size limitation andirrespective of applied magnetic field orientation.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art having knowledge of various magnetoresistantdevices, their use and fabrication. Such objects, features, benefits andadvantages will be apparent from above as taken into conjunction withthe accompanying examples, data, figures and all reasonable inferencesto be drawn there from, alone or with consideration of the referencesincorporated herein.

In part, the present invention can be directed to a junction devicecomprising a heterojunction composite. Such a device can comprise acomposite comprising a p-type Group III-V ferromagnetic semiconductorcomponent comprising a compound of a formula

(M^(III) _(1-x)M^(TM) _(x))M^(V)

wherein M^(III) can comprise a Group III metal component, M^(TM) cancomprise a magnetic transition metal component, and M^(V) can comprise aGroup V element component. Such a composite can also comprise an n-typeGroup III-V non-magnetic semiconductor component comprising a compoundof a formula M^(III)M^(V)). Without limitation, x can be less than about0.2; a Group III metal component can be selected from Al, Ga and In; aGroup V element component can be selected from P, As and Sb; and atransition metal component can be selected from Cr, Fe and Mn. Incertain embodiments, such a device can comprise a ferromagneticcomponent comprising (In_(1-x)Mn_(x))As, and a non-magnetic componentcomprising InAs. Various other Group III-V ferromagnetic semiconductorcompounds can be as are described more fully in co-pending applicationSer. No. 10/698,352, filed Oct. 31, 2003, the entirety of which isincorporated herein by reference.

As illustrated below, heterojunction composites of this invention cancomprise a ferromagnetic component epitaxial with respect to anon-magnetic component. As representative results indicate, suchcomposites can be utilized and/or fabricated with a range ofmagnetoresistive devices, including but not limited to magnetodiodes.With regard to the latter, composite compositions and/or configurationsof this invention can be used to fabricate such a device dimensioned toincrease current density and magnetoresistance responsive to an appliedmagnetic field.

In part, this invention can also be directed to a method of monitoringmagnetoresistance. Such a method can comprise providing a device of thesort more broadly described above; applying a voltage across the device;applying a magnetic field oriented with respect to a current through thedevice; and measuring magnetoresistance of such a current responsive tothe applied magnetic field. Alternatively, as would be understood in theart, such a method can be used to measure voltage change upon applying acurrent. Regardless, in certain embodiments thereof, as can be achievedwith various such device structures, magnetoresistance can increase withincreasing magnetic field. Demonstrating such a relationship,magnetoresistance can be measured at a temperature from about 25 K toabout 300 K. Regardless of temperature, various such embodiments canshow magnetoresistance increasing substantially linearly with a magneticfield increasing from about 1.5 T to about 9 T. Such benefits associatedwith this invention can be especially advantageous at temperatures up toabout or higher than 300 K. Likewise, various such embodiments of thisinvention demonstrate that the magnitude of a linear increase inmagnetoresistance can increase with increasing applied current. Suchbenefits and results can be especially useful at high magnetic fieldstrengths (e.g., up to about 9 T). Regardless, in conjunction with oneor more of the device structures described herein, such a monitoringmethod can be enhanced by decreasing device dimension, to increasecurrent density and increase magnetoresistance.

In part, this invention can also be directed to a magnetodiode devicecomprising a p-type ferromagnetic semiconductor component comprising acompound of a formula

(In_(1-x)Mn_(x))As

wherein x can be greater than zero and less than about 0.02; and ann-type non-magnetic semiconductor component comprising a compound of aformula InAs. As illustrated below, such a device can comprise anepitaxial ferromagnetic film on, coupled to and/or deposited on anon-magnetic substrate. Regardless of ferromagnetic stoichiometry, sucha device can be dimensioned to increase current density andmagnetoresistance responsive to an applied magnetic field.

This invention can also be directed to a method of using a Group III-Vferromagnetic semiconductor/Group III-V non-magnetic semiconductorheterojunction to detect an applied magnetic field. Such a method cancomprise providing a device structure comprising a(In_(1-x)Mn_(x))As/InAs heterojunction composite, such a devicepositioned in an applied magnetic field; applying a voltage across thedevice; and detecting magnetoresistance of the heterojunction responsiveto the applied magnetic field, such magnetoresistance positive undermagnetic fields applied both parallel to and perpendicular to currentflow through the device. As would be understood by those skilled in theart, such a method can be especially useful with a field applied atabout room temperature (e.g., up to about and above 300 K). Regardless,using such a method, heterojunction magnetoresistance can besubstantially linearly dependent on such an applied field. Likewise,without limitation as to temperature or field strength, such a methodcan be employed to detect an applied magnetic field, regardless ofangle; that is, whether such a field oriented from about parallel toabout perpendicular to current flow through the device. Accordingly, incertain such embodiments of this invention, one such heterojunctivedevice can detect an applied magnetic field irrespective of its angularorientation to current flow.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1. Current density-voltage characteristics for anIn_(0.96)Mn_(0.04)As/InAs junction at 78 and 294 K.

FIG. 2A-B. Forward bias J-V characteristics of an In0.96Mn0.04As/InAsjunction at 78 K (A). The ideality factor as a function of voltage,calculated after subtracting the linear leakage current from the J-Vcharacteristics, is shown in (B).

FIG. 3. The ideality factor as a function of voltage for anIn0.96Mn0.04As/InAs junction at varying temperatures.

FIG. 4. The longitudinal magnetoresistance (%) as a function of voltagefor an In0.96Mn0.04As/InAs junction at 78, 180, and 295 K.

FIG. 5A-B. The magnetic field dependence of the percentmagnetoresistance (A) in the low bias, leakage regime and (B) in thehigh bias (0.335 V), diffusion regime for an In0.96Mn0.04As/InAsjunction at 78 K. The solid line in (a) shows the H1.7 dependence, whilethe solid line in (B) shows the linear magnetic field dependence.

FIG. 6A-B. Schematic diagrams of magnetodiodes, illustrating two typesof InAs contact, in accordance with this invention. Component dimensionsare illustrative, only.

FIG. 7. Schematic representation of a of the p-In0.96Mn0.04As/n-InAsmesa diode. As above, component dimensions are non-limiting.

FIG. 8. Forward bias I-V characteristics at 295 K in the presence ofmagnetic fields ranging from 0-8 T.

FIG. 9 A-B. Percent magnetoresistance as a function of magnetic field at25, 78, and 295 K (A). Room temperature voltage as a function ofmagnetic field at I=5, 11, and 15 mA (B).

FIG. 10. Magnetoresistance as a function of forward bias at 295 K and 1T. The solid line shows a fit to the data using eq. (5).

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

This invention can be directed to new magnetoresistive devices, relatedcomposites and component materials and/or methods relating to the use ofa Group III-V ferromagnetic semiconductor and a Group III-V nonmagneticsemiconductor heterojunction. For instance, such magnetodiodes exhibit alarge junction magnetoresistance that is linearly dependent on theapplied magnetic field at room temperature. As a result, this inventionhas applications in magnetic field sensors, gaussmeters, and othermagnetoresistive devices, and can be readily integrated into currentIII-V semiconductor circuitry, enabling e.g., magnetic imaging.Accordingly, such devices can, without limitation, be used in a range ofapplications in which a continuous change in current is desired withincreasing magnetic field. This advantage can be utilized, for instance,in conjunction with high field gaussmeters.

Without restriction to any one embodiment, a magnetodiode device of thisinvention can comprise a ferromagnetic p-type In1-xMnxAs thin filmdeposited (e.g., by metalorganic vapor phase epitaxy) on an n-type InAssubstrate. (See, e.g., FIG. 6.) When such a device is operated in theturn-on region in forward bias, the diode current is strongly dependenton the presence of magnetic fields. Magnetic fields applied eitherparallel (longitudinal) or perpendicular (transverse) to the flow ofcurrent through the devices lead to a positive magnetoresistance. Thisallows for one device/sensor to measure magnetic fields in alldirections, whereas with conventional magnetodiodes at least two sensorswould be required for multi-directional sensing. The observed positivemagnetoresistance is in contrast to the negative magnetoresistance thatwas predicted in the art to arise in ferromagnetic/nonmagneticsemiconductor diodes. Magnetoresistance is linearly dependent on themagnetic field for fields greater than 1000 Oe and 2500 Oe, for certaindevices operated at 78 K and 295 K, respectively. Such a linear responseindicates this invention can be used in conjunction with magnetic fieldsensors, gaussmeters, and other magnetoresistive devices. Devicestructures and configurations illustrated herein are provided only forpurpose of illustration. Other such structures and componentconfigurations would be understood by those skilled in the art madeaware of this invention.

The electronic and magnetotransport properties of epitaxialp-(In,Mn)As/n-InAs heterojunctions were examined in conjunction withthis invention. As described below, the junctions were formed bydepositing ferromagnetic (In,Mn)As films on InAs (100) substrates usingmetal-organic vapor phase epitaxy. The current-voltage characteristicsof the junctions were measured from 78 to 295 K. At temperatures below150 K, ohmic currents appear to dominate transport at low bias, followedby defect-assisted tunneling current with increasing bias. At highforward bias, junction transport appears dominated by diffusion current.The magnetoresistance of the junctions were measured as a function offorward bias and applied magnetic field. Without limitation, themagnitude and field dependence of the longitudinal magnetoresistanceappear to depend, at least in part, on the junction transport mechanism.Under high bias, a magnetoresistance of 15.7% at 78 K and 8% at 295 K ina 4400 Oe field was measured in an In0.96Mn0.04As/InAs junction. At 78K, the high bias magnetoresistance increases linearly with magneticfield from 1000 to 4600 Oe.

The current density-voltage characteristics of an In0.96Mn0.04As/InAsjunction at 78 and 294 K are shown in FIG. 1. Diode behavior isobserved. At temperatures below 220 K, a linear leakage current isobserved under reverse bias. This current also dominates the junctioncharacteristics at low forward bias, as will be discussed below.

The forward bias current density-voltage characteristics of theIn_(0.96)Mn_(0.04)As/InAs junction measured at 78 K are shown in FIG. 2(a). The total forward bias current density is given by:

J=J _(diff) +J _(gr) +J _(tun) +J _(leakage),   (1)

where J_(diff) is the diffusion current, J_(gr) is thegeneration-recombination current, J_(tun) is the tunneling current andJ_(leakage) is the leakage current The measured J-V characteristics werefit to the expression,

$\begin{matrix}{{J = {J_{0}{\exp \left( \frac{qV}{\eta \; k_{B}T} \right)}}},} & (2)\end{matrix}$

where η is the ideality factor defined by the expression,

$\begin{matrix}{{\eta = {\left( \frac{q}{k_{B}T} \right)\left( \frac{V}{{\ln}\; I} \right)}},} & (3)\end{matrix}$

At low bias, a linear J_(leakage) component dominates junctiontransport, indicating the presence of parallel conduction paths shuntingthe junction. These parallel conduction paths are believed to arise fromsurface states created during the mesa etching process. Aftersubtracting this ohmic component from the total current, the idealityfactor was obtained. Ideality factors are commonly used to determine theorigin of diode currents: junction conduction is dominated by diffusionwhen η=1, generation and recombination (or diffusion under highinjection conditions) when η=2, and defect-assisted tunneling when η>2.The ideality factor for the In_(0.96)Mn_(0.04)As/InAs junction as afunction of bias is shown in FIG. 2(B). The ideality factor increasesfrom 2.2 to 10.6 as the bias is increased from 0.145 to 0.255 V,respectively. This suggests that the conduction mechanism changes fromgeneration and recombination to defect-assisted tunneling as the bias isincreased. Conduction from the diffusion current component begins toincrease at biases greater than 0.28 V, leading to a decrease in theideality factor. The ideality factor reaches a local minimum at 0.32 Vof η=2.6. This ideality factor is greater than the high injection valueof η=2 presumably due to series resistance and continued tunnelingcurrents, both of which can increase the ideality factor. The increasein ideality factor at biases greater than 0.32 V can be attributed toseries resistance.

The temperature dependence of the ideality factor for theIn0.96Mn0.04As/InAs junction is shown in FIG. 3. The maximum value ofthe ideality factor decreases with increasing temperature from 10.6 at78 K to 2.9 at 140 K. The minimum value of the ideality factor arisingfrom diffusion currents also decreases with temperature from 2.6 at 78 Kto 1.8 at 140 K. These results indicate that the diffusion currentcomponent increases as the temperature increases. Similar transportbehavior was also observed in In1-xMnx,As/InAs junctions with x=0.035and x<0.01. At room temperature, the ideality factors of the all diodesmeasured ranged from 1.3 to 1.7, indicating that the diffusion andgeneration-recombination currents dominate junction transport.

The current-voltage characteristics have been measured in the presenceof an applied magnetic field. FIG. 4 shows the percent magnetoresistanceas a function of voltage for the In0.96Mn0.04As/InAs junction with a4400 Oe field applied parallel to the direction of current flow. Thepercent magnetoresistance is given by 100×(R(H)−R0)/R0; a decrease inthe junction current corresponds to a positive magnetoresistance. Themagnitude of the magnetoresistance is directly dependent on the junctiontransport mechanism. At 78 K, a constant magnetoresistance of 1.7% isobserved under low bias from 0.01 to 0.15 V, where the transport isdominated by ohmic leakage currents. The magnetoresistance begins toincrease with the onset of tunneling currents at 0.16 V, reaching 6% at0.27 V. The magnetoresistance remains at roughly 6% until the bias isincreased past 0.31 V, at which point the magnetoresistance increasesrapidly with increasing forward bias. The onset of the second increasein magnetoresistance corresponds to the local minimum in ideality factorand accordingly to the diffusive transport regime. The large observedmagnetoresistance is equal to 15.7% at 0.4 V. Similar magnetoresistancebehavior was also measured in a junction with x<0.01. At 180 K, only twomagnetoresistance regimes are present as tunneling currents are notobserved. The magnetoresistance is constant from 0.01 to 0.1 V. A rapidincrease in magnetoresistance is observed as the bias is increased past0.1 V, which corresponds to the onset of diffusion current. Amagnetoresistance of 8% at 0.15 V is observed at room temperature.

The field dependence of the magnetoresistance is given in FIG. 5, asmeasured at 78 K in an In0.96Mn0.04As/InAs junction. Themagnetoresistance is subquadratic and follows a H1.7 power lawdependence in the leakage current regime, as shown in FIG. 5(A). At highbiases, where diffusion currents dominate transport, themagnetoresistance is linearly dependent on the applied magnetic fieldfrom 1000 to 4600 Oe, as shown in FIG. 5(B). A linear magnetoresistancein the diffusive current regime is also observed at 78 K in the junctionwith x<0.01, while the low bias magnetoresistance exhibits a H1.8dependence.

The magnetoresistance observed at low bias, in the ohmic conductionregime, is attributed to Lorentz scattering of carriers throughout thediode. See, R. A. Stradling and P. C. Klipstein, Growth andcharacterisation of semiconductors (Hilger, Bristol, England; New York,1990). As the magnetic and electric fields are parallel a negligiblemagnetoresistance might be expected. However, longitudinalmagnetoresistance is often observed in semiconductors due to defects,dislocations or electrical inhomogeneities that scatter carriers. Themeasured field dependence of Hγ, where γ ranges from 1.7 to 1.85, is inrelative agreement with the expected H2 dependence of magnetoresistancedue to Lorentz scattering. See, S. M. Sze, Physics of semiconductordevices (Wiley, New York, 1981).

The origin of the unusual linear dependence of the magnetoresistance athigh bias is currently under study. Nevertheless, the fact that themagnitude and field dependence depends on the specific injectionmechanism suggests that the behavior is junction related. The measuredmagnetoresistance is not attributed to an increased resistance in thebulk InAs substrate or the InMnAs film. The longitudinalmagnetoresistance of an InAs substrate and an InMnAs film was measuredin the same configuration as the (In,Mn)As/InAs junctions. In bothcases, the measured magnetoresistance was less than 1% and independentof applied voltage.

The observed positive magnetoresistance is counter to themagnetoresistance predicted for a ferromagnetic/nonmagneticsemiconductor junction for diffusive transport. A negativemagnetoresistance is predicted with the increase in current proportionalto exp[(Δ(H)−Δ(H=0))/k_(B) T], where A(H) is the conduction band-edgesplitting due to a magnetic field H. The fact that a positivemagnetoresistance is observed suggests that the conduction bandsplitting in (In,Mn)As is small and that another magnetoresistancemechanism, the origin of which is presently unknown, plays a greaterrole in magnetotransport.

With reference to examples 3-5, the measured magnetoresistance can bedescribed using the diode equation and adding a series magnetoresistanceterm R(H) to the argument. The I-V characteristics of theIn_(0.96)Mn_(0.04)As/InAs junction are modeled using the equation:

$\begin{matrix}{{I = {I_{0}{\exp \left( \frac{V_{A}}{\eta \; k_{B}T} \right)}\left( \frac{- {IR}_{0}}{\eta \; k_{B}T} \right)\left( \frac{- {{IR}(H)}}{\eta \; k_{B}T} \right)}},} & (4)\end{matrix}$

where η is the ideality factor, R₀ is the zero-field series resistance,and R(H) is the magnetic field dependent series resistance. From the I-Vcharacteristics, the experimental values of I₀, R₀, R(H) and η wereobtained. The ideality factor and R(H=1 T) were fixed at theexperimentally obtained values of η=1.41 and R=1.14Ω, respectively,while I₀ and R₀ were varied to achieve the best fit. FIG. 10 shows thefit to the magnetoresistance as a function of applied bias at 1 T and295 K. (See, example 4 and eq. 5). The values of I₀ and R₀ obtained fromthe fit, 9×10⁻⁴ A and 1.37Ω respectively, are in good agreement with theexperimentally obtained values of 5.9×10⁻⁴ A and 1.12Ω.

One possible origin of the magnetoresistance is carrier scattering dueto fluctuations and clustering of the Mn ions at or near the junction.Parish and Littlewood predicted that mobility disorder can result in anon-saturating linear magnetoresistance in semiconductors. Withoutrestriction to any one theory or mode of operation, an inhomogeneousdistribution of magnetic Mn ions could cause fluctuations in thescattering rate of injected electrons, giving rise to such localvariations in carrier mobility. While the model of Parish and Littlewoodhas not been applied to p-n junctions, it should be noted that inAg_(2+δ)Te a linear magnetoresistance emerged when the electron and holeconcentrations in the material were equivalent. This is consistent withthe present results in that the linear magnetoresistance inp-(In,Mn)As/n-InAs junctions occurs only under high injection conditionswhen the electron and hole concentrations near the depletion region areequivalent. Further, extended x-ray absorption fine structure (EXAFS)measurements have provided evidence that Mn ions cluster into nearestneighbor cation sites forming dimers and trimers in MOVPE grown InMnAsfilms.

EXAMPLES OF THE INVENTION Example 1

P-type In_(1-x) Mn,As films were deposited at 480° C. by atmosphericpressure metal-organic vapor phase epitaxy on nominally undoped, n-typeInAs (100) substrates to form the heterojunctions. Precursors used weretrimethylindium (TMIn), 0.3% arsine (AsH₃) in hydrogen andtricarbonyl(methylcyclopentadienyl)manganese (TCMn). A pre-growth annealat 510° C. was carried out under an arsine overpressure in order toremove surface oxide from the InAs substrate. A detailed description ofthe growth conditions for (In,Mn)As films has previously been reported.See, A. J. Blattner, J. Lensch, and B. W. Wessels, Journal of ElectronicMaterials 30, 1408 (2001). Manganese concentrations were determinedusing standards-based energy dispersive x-ray spectroscopy (EDS). X-raydiffraction was used to verify phase purity of the (In,Mn)As films. Filmthickness, as determined by profilometry, ranged from about 100-about525 nm. Room temperature hole concentrations of (In,Mn)As films are onthe order of 10¹⁸-10¹⁹ cm⁻³. The room temperature electron concentrationof the InAs substrates was 1.4×10¹⁷ cm⁻³. Magneto-optical Kerr effect(MOKE) measurements indicated that the In_(0.96)Mn_(0.04)As andIn_(0.965)Mn_(0.035)As layers were ferromagnetic at room temperature.

Example 2

Mesa diodes were fabricated from the epitaxial structures usingconventional photolithography and a citric acid wet etch. See, G. C.Desalvo, R. Kaspi, and C. A. Bozada, J. Electrochem. Soc. 141, 3526(1994). The mesa diameter was 250 μm. Ti/Au (15/175 nm) ohmic contactswere evaporated to the (In,Mn)As films. Silver paste was used to makecontact to the InAs substrates. The diodes were wire bonded to anon-magnetic chip carrier with gold wire. Low-temperature measurementswere carried out in a Janis ST-100 cryostat with a non-magnetic sampleholder. A Keithley 2400 source-meter was used to source voltages andmeasure currents. Magnetoresistance measurements were made in fields ofup to 4600 Oe applied parallel to current flow across the junction.

Using a similar approach, high field, longitudinal magnetoresistancemeasurements were taken on a p-In_(0.96)Mn_(0.04)As/n-InAs junction. Anonsaturating linear magnetoresistance at fields greater than 1.5 T wasobserved at room temperature. The measured magnetoresistance is welldescribed by a modified diode equation with a magnetic field-dependentseries resistance.

Example 3

For these studies, a 500 nm thick p-In_(0.96)Mn_(0.04)As layer wasdeposited on an undoped n-InAs substrate using atmospheric pressuremetal-organic vapor phase epitaxy under conditions previously described.(A. J. Blattner, J. Lensch, and B. W. Wessels, J. Electron. Mater. 30,1408 (2001).) The room temperature electron concentration of the InAssubstrate is 2×10¹⁶ cm⁻³, while the room temperature hole concentrationof the p-InMnAs film is on the order of 10¹⁸−10¹⁹ cm⁻³. The diodes werepatterned into 250 μm diameter mesas using photolithography and wetetching. Ti/Au was used as the p-In_(0.96)Mn_(0.04)As ohmic contact,while Ag was used as the n-InAs ohmic contact. Magnetoresistancemeasurements were made with the magnetic field applied parallel to theflow of current through the mesa diode using a Quantum Design PhysicalProperties Measurement System. A schematic of the experimental setup isshown in FIG. 7.

Example 4

FIG. 8 shows the room temperature I-V measurements made with appliedmagnetic fields over the range of 0 to 8 T. The junction becomes moreresistive with increasing magnetic field and shows no sign of saturatingwith field. The magnetoresistance, MR, defined as

$\begin{matrix}{{{MR} = {\left( {\frac{{V(H)}}{I} - \frac{{V\left( {H = 0} \right)}}{I}} \right)/\left( \frac{{V\left( {H = 0} \right)}}{I} \right)}},} & (5)\end{matrix}$

is shown in FIG. 9A as a function of applied field for a constantcurrent of 11 mA at 25, 78 and 295 K. The room temperaturemagnetoresistance increases linearly with field from 1.5 to 9 T. Themagnitude of the linear magnetoresistance increases with increasingapplied current from 360% (5 mA) to 710% (15 mA) in a 9 T field. Thefield dependence of the magnetoresistance becomes sublinear withdecreasing temperatures. At 78 K the magnetoresistance is proportionalto Ho^(0.84), while at 25 K the magnetoresistance is proportional toH^(0.64).

Example 5

Due to its large room temperature magnetoresistance, this device holdspromise for magnetic sensing applications. (H. P. Baltes and R. S.Popovic, Proc. IEEE 74, 1107 (1986); R. S. Popovic, Hall Effect Devices(Institute of Physics, Bristol; Philadelphia, 2004).) The change involtage as a function of magnetic field for three operating currents atroom temperature is shown in FIG. 9B. For all currents the change involtage is linear with increasing magnetic field from 1.5 to 9 T.

As shown above and in the accompanying figures, the current-voltagecharacteristics of p-(In,Mn)As/n-InAs heterojunctions have been measuredover the temperature range of 78 to 295 K. Three forward bias transportcomponents are observed in the junctions at temperatures below 150 K.Ohmic leakage currents are observed at low bias, defect-assistedtunneling currents at intermediate bias and diffusion currents dominateat high bias. A positive magnetoresistance is observed with theapplication of a longitudinal magnetic field. The magnitude and fielddependence of the magnetoresistance depend on the junction transportmechanism. A magnetoresistance of 15.7% and 8% is observed under highbias at 78 and 295 K, respectively. At 78 K, the high biasmagnetoresistance increases linearly with increasing magnetic field from1000 to 4600 Oe.

The high field magnetoresistive properties of ap-In_(0.96)Mn_(0.04)As/n-InAs junction were also measured. Under forwardbias, a large, nonsaturating magnetoresistance is observed attemperatures from 25 to 295 K in fields up to 9 T. At room temperature,the magnetoresistance increases linearly with magnetic field from 1.5 to9 T and is greater than 700% at 9 T. As shown, the magnetoresistance canbe simulated using a modified diode equation, including afield-dependent series magnetoresistance.

We claim:
 1. A method of monitoring magnetoresistance, said methodcomprising; providing a junction device comprising a heterojunctioncomposite comprising a p-type Group III-V ferromagnetic semiconductorcomponent comprising a compound of a formula(M^(III) _(1-x)M^(TM) _(x))M^(V) and an n-type Group III-V non-magneticsemiconductor component comprising a compound of a formula(M^(III)M^(V)), wherein M^(III) comprises a Group III metal component,M^(TM) comprises a magnetic transition metal component and M^(V)comprises a Group V metal component, and x is less than about 0.2;applying a voltage across said device; applying a magnetic fieldoriented with respect to a current through said device; and measuringmagnetoresistance of said current responsive to said applied magneticfield.
 2. The method of claim 1 wherein said magnetoresistance increaseswith increasing magnetic field.
 3. The method of claim 2 wherein saidmagnetoresistance is measured at a temperature from about 25 K to about300 K.
 4. The method of claim 3 wherein said magnetoresistance increasessubstantially linearly with a magnetic field increasing from about 1.5 Tto about 9 T.
 5. The method of claim 4 wherein the magnitude of saidlinear increase in magnetoresistance increases with increasing appliedcurrent.
 6. The method of claim 1 wherein decreasing said devicedimension increases current density and increases magnetoresistance.